Today's high-performance wireless digital communication systems are based on a radio access method called code division multiple access (CDMA). A particular extension to CDMA is known as wideband CDMA (WCDMA) which is an access method standardized within the 3G partnership project (3GPP). The WCDMA standard is the basis for 3G cellular systems. WCDMA systems provide higher user data rates (bits/sec) than existing CDMA systems. The current WCDMA standard offers up to 2 Mbps user data rate, and even higher data rates are envisioned in the future. A new mode of operation referred to as high speed downlink shared channel (HSDPA) is currently being standardized by 3GPP. HSDPA offers user data rates of up to 10–15 Mbps per user. Such high data rates as those utilized within WCDMA systems impose large demands on the digital receiver architecture of base stations and mobile stations in terms of accuracy and speed. The additional requirements for mobile equipment, as compared to receivers in stationary equipment, include a need for low power consumption and low production cost. These requirements are often contradictory to the new higher data rates.
One solution has been the use of direct conversion (homodyne) receivers. A direct conversion (homodyne) receiver is a high-performance, cost-effective alternative to the traditionally used superheterodyne receivers. However, the disadvantage with direct conversion (homodyne) receivers is that they generate a DC offset. This offset must be cancelled before data detection is commenced because the DC offset will severely degrade the receiver's performance. Many different solutions have been proposed to suppress the DC-components, and the majority of these methods use some kind of averaging to estimate the DC offset within the signal.
A WCDMA system features, as one of the multitude of channels implemented within the system, a common pilot channel (CPICH) used for, for example, channel estimation. A WCDMA system implements a spreading factor (SF) for CPICH of 256 which suppresses the receiver-generated DC offset during the despreading process. The despreading process DC-suppressing ability is proportional to the spreading factor, and, hence, the DC offset is suppressed 24 dB (a factor of 256) when the CPICH is despread.
Each dedicated channel of the WCDMA system also features a number of pilot symbols (dedicated pilot symbols). A user data rate implies a rather low spreading factor on dedicated channels, (i.e., down to SF=4). The low spreading factor implies that the DC offset suppression due to despreading is relatively low, and the dedicated pilot-symbol estimates will contain a DC offset residue. Thus, there is a need for DC offset suppression techniques within high-speed WCDMA systems.